Anything offchain has a whole bunch of issues that are either naively or deliberately obscured by the fact that it _eventually_ writes to the blockchain. The exchanges that offer instant settlement are circumventing trust by doing the settlement for you. You get speed, but not security that they have done what they have said they have.
To be honest I think the issue here is not due to speed of settlement, but layer-2 is not an acceptable substitute because it does not allow reversability. For the merchants it's good that they are getting the money right away, but most consumers will not dare to pay anything via layer-2 networks simply because they won't have any recourse in case they want to undo the transaction.
How long it's going to take you to realize that even if we built everything you are asking for, we are STILL going to end up with a system that is not as capital-efficient as the status quo?